Apparently, you can not kill them unless you bring them up and bury them or inject them with some lethal concoction. Cutting them up may allow them to multiply faster since they can regererate legs and half bodies can regenerate into two COTs. Both methods of killing them noted above have some significant risks involved so I just usually push them off the reef with a metal pointer and make them start their climb again. I know this only slows them down but....
FYI, just moving them has it's own risks. One dive saw me move a COT with a pointer. He figured that he should help the reef too so he grabbed one with his bare hand to move it. He found out how silly that was very quickly. Ever since then, if I'm in an area with COT, I alwasy make sure to point out to every diver what they are and how dangerous they are.